Masteries

Offense:
21

Defense:
0

Utility:
9

I find this build to be great no matter where you're laning.

Notes:
-Tristana's passive ability gives her greater range as she levels up, which gives her unparalled range for auto-attack by higher levels.
-Use Rocket Jump defensively unless you're sure you can get a kill, which eliminates the CD on Rocket Jump so you can get out of melee range.
-Rocket Jump let's you bypass walls to escape, to cut someone off or even to quickly get into (or out of) the enemy base.
-Rocket Jump slows enemies you land on!
-Explosive Shot is an excellent harass.

Masteries:
Self explanatory for the most part, they focus on damage output, armour penetration and crits. I find the point in Ignite (+10 AP while Ignite is on cooldown) more helpful than a .75% CD reduction, as everything but Rapid Fire benefits slightly.

Summoner Spells:Teleport: Useful at the beginning of the game to get back to your tower quickly if you go back to your base to get items, health or if you died. Mid-game and late game it's perfect to get to a tower quickly to protect it, or to teleport near an ally for an unexpected gank. Most people don't see the swirling lines around your minion when you teleport to it. Late game, ethical implication aside, it can be used to get near the enemy's base to quickly backdoor.

Ignite: Use with caution, as you lose the 10 AP bonus while it's on CD. Don't use it to harass, use it to scare someone off or to make sure the enemy dies. It's perfect to use in conjecture with Explosive Shot, as by midgame the combination of the two will deal ~525 dmg over 7 seconds while reducing healing and health regeneration by 50%.

Runes:
29.37 armour penetration from Marks and Quintessences is awesome. This let's you efficiently deal with tanks early game, and stacks nicely with Black Cleaver later on.
For Seals, I use crit chance (3.78%) rather than crit dmg because I enjoy consistently dealing dmg and Seals are more effective with crit chance than crit dmg.
For Glyphs, I use crit dmg (5.04%) because the extra crit dmg is much better than the crit chance Glyphs.

Items/Gameplay:Early Game:
If you're mid, grab Boots of Speed, if not, grab the Dagger. The speed boost is important mid-lane because it makes both harassing, fleeing and dodging skills shots easier. If you can, attack the mage minions in the back, as it usually scares your opponent or brings them into range of Explosive Shot. Use Explosive Shot at every opportunity, if they just used a potion, they will only receive 50% healing from it. It's a relatively subtle DoT as it doesn't deal tons of dmg, but it lasts quite a while at higher lvls. Make sure to always keep enough mana to Rocket Jump out of danger in case they Flash or slow you and charge.

On your first trip back, you should be able to grab Berserker's Boots and maybe even a Longsword. Teleport back if it's urgent or there's an opporunity to kill someone. If not, enjoy the walk with your newfound movement speed!
If your enemy has ~400 hp (when you're lv 4 or so), Rocket Jump onto them, Blow Explosive Shot, Ignite and Rapid Fire. Just repeatedly shoot them in the face, and they're bound to go down. Unless they have cleanse, they're screwed. Heal will negate Explosive Shot's overall dmg, but you can usually put out enough dmg to kill them.

The second or third time you head back to base, you should have be able to afford Sword of the Occult. Before you buy it, think for a moment. Does it seem like you're going to be killing a lot more than dying? If so, definitely get it. I like to get it early game, because you'll benefit from it soon, or be able to have more stacks when needed. If you don't think you can kill enough to make it worthwhile, start working on Phage instead.

Mid-game:
When people start leaving their lanes (usually around lv 8-10) it is officially mid-game. If you got Sword of the Occult, you should have 2-6 stacks. Regardless of what item you got, you should work on Black Cleaver. The 75 dmg is great, and the -12 armour/atk stacks perfectly with your runes and masteries. Tanks like Sion, Dr. Mundo or Cho'Gath that normally spearhead an enemy charge can be wiped out quickly by your team because you can sunder 60 armour which leaves them very susceptible to focus fire, while you ignore up to 95.37 armour. Most tanks have ~175 armour (~64% physical dmg reduction) which means that when you attack them, you'll fight them as if they only had ~80 armour (~44% dmg reduction).

There's nothing special to do mid-game. Help out other lanes when you see ganking opportunities. Keep last hitting minions. Grab the lizard buff whenever you can. Get Phantom Dancer. Remember that Explosive Shot makes enemies explode upon you last hitting them, dealing up to 150 magic dmg to nearby minions and champions. When you find yourself pushing a tower, remember to use Rapid Fire to grant a huge AS bonus, but watch the mini-map to make sure you're not about to get ganked.

Late Game:
At lv 18, your passive almost gives you enough range to attack turrets without them being able to target you. This means that whenever an opposing champions is tower hugging, and isn't behind the tower, you can hit them without being targeted by the turret! You can also shoot enemies from behind trees or base walls! Now is when you shine the most. You outrange everyone except for TF's Wild Cards and Ezreal's Mystic Shot. Use Buster Shot sparingly, as its pushback can save you or an ally. Use it not for the dmg, but tactically.

You should have anywhere from 8-20 stacks on Sword of the Occult by lv 18 or you should have Phage. You should also have Berserker's Boots, Black Cleaver, Phantom Dancer and be working on Bloodthirster. Once you do get Bloodthirster, go lane for a while, or jungle. Every kill you get gives you +1 dmg and .25% lifesteal (up to 40 dmg and 10% lifesteal) until you die.

Conclusion:
Tristana deals brutal dmg if she is played right. Make sure you're one of the people who plays her right.

If your team has no DPS or nukers, get Infinity Edge instead of Phage/Frozen Mallet. They should have enough CC, but they'll need dmg.

If your team has a lot of DPS or nukers on it, get frozen mallet right after the boots, as it'll let you receive more of a beating and also slow anything you hit by 30% for 2.5 seconds.

If you're dominating early on, don't get cocky. Running around by yourself will eventually get you repeatedly ganked. LoL is a team game and you're stuck with your team no matter how much you don't want to be. A bunch of mediocre players who work as a team do much better than a team of 'good' players who refuse to cooperate with one another.